This invention relates to a process for controlling the final quality of synthetic polymeric yarns during manufacture thereof.
Conventionally, the quality of yarn is determined by taking final product yarn samples and measuring chemical and physical properties of the samples in the laboratory. One yarn property that is measured is relative viscosity of the yarn. The term "relative viscosity" as used herein is defined as the viscosity of a dilute solution of the polymer divided by the viscosity of the solvent employed. For polyamides, the standard measurement of relative viscosity is the viscosity of an 8.4 wt. % solution of the polyamide in a 90 wt. % formic acid solution divided by the viscosity of the 90 wt. % formic acid.
The laboratory measurement of relative viscosity may be used to make adjustments to process conditions if the relative viscosity measurements differ from desired relative viscosity levels. The disadvantage with this process is that the lag time between the laboratory measurement and the adjustments is generally too long to provide meaningful, effective control of the process.
U.S. Pat. No. 4,675,378 discloses a process control scheme for controlling the quality of a polymer by controlling melt viscosity. Control is effected by continuously measuring the melt viscosity at the outlet of the polymerization vessel. Operating conditions in the polymerization vessel are adjusted when the viscosity at this point strays from the pre-established level. Melt viscosity is also controlled by periodically measuring viscosity just before the spinneret. When this viscosity level deviates from a pre-set level, the pre-established viscosity level in the reaction vessel is adjusted.
It is believed that this process is inaccurate since it suffers from several disadvantages. One disadvantage is that inferring melt viscosity at finite locations such as at the exit of the polymerization vessel or at some point before the spinneret does not take into account changes that may take place in the material before it is actually formed into fibres.
Another disadvantage is that the pressure measurements taken by the viscometers of this process are taken close together. Pressure drops measured over short distances may give a good representation of the polymer viscosity at a precise location However, such pressure drops are typically relatively small, therefore the measurement of these pressure drops is less precise.
One approach that has been taken is to increase pressure drop and thus improve the accuracy of such viscometers. This may be done either by diverging a side stream of polymer through a smaller orifice or by increasing the flow rate of this side stream through the use of an additional pumping means. This approach may cause a number of problems. For example, the higher shear rates created by these methods of increasing the pressure may induce an increase in temperature. Also, at the required shear rate, the polymer may exhibit non-newtonian flow properties instead of newtonian flow properties, which may effect the reliability of the results.
It is an object of the present invention to obviate or mitigate the above-mentioned disadvantages.
Accordingly, the invention provides a process for controlling the relative viscosity of synthetic yarns during production thereof comprising the steps of:
taking at least three pressure measurements at three spaced locations in a transfer line for carrying molten polymer to a spinneret;
measuring temperature and throughput of polymer in said transfer line;
calculating an estimated relative viscosity of yarn produced by employing a predetermined correlation between pressure drop, throughput, temperature and relative viscosity; and
adjusting a meaningful operating condition in said melting zone in response to deviations in the estimated relative viscosity of said yarn from a desired relative viscosity of said yarn.
In another one of its aspects the invention provides a process for controlling the relative viscosity of synthetic yarns during production thereof comprising the steps of:
measuring the temperature and throughput of said yarn;
taking at least three pressure measurements at three spaced locations in a transfer line for carrying molten polymer to a spinneret;
calculating an estimated relative viscosity of yarn produced by employing the equation: ##EQU1## wherein: RV=relative viscosity
THPT=throughput of polymer PA0 dP1=P1 - P2 PA0 dP2=P3 - P2 PA0 P1,P2,P3 pressure measurements taken ##EQU2## wherein:
Ea=activation energy for viscosity
R=ideal gas constant
T=absolute temperature of process
and
adjusting a meaningful operating condition in said melting zone in response to deviations in the estimated relative viscosity of said yarn from a desired relative viscosity of said yarn.
In another one of its aspects, the invention provides an apparatus for controlling the quality of yarn produced during manufacture thereof comprising:
at least three pressure sensors locatable within a transfer line for transferring molten polymer to a spinneret;
pressure measuring means and throughput measuring means associated with said transfer line for respectively measuring pressure and throughput of polymer therethrough;
calculation means for determining an estimated relative viscosity of said yarn employing a correlation between, temperature and throughput measurements made by said temperature measuring means and said throughput measuring means respectively, pressure measurements made by said pressure sensors and said estimated relative viscosity; and
control means associated with said calculation means for adjusting a meaningful operating condition in said polymerization vessel when said estimated relative viscosity deviates substantially from a desired relative viscosity of said yarn.
In the present invention, the relative viscosity of the yarn is estimated using a correlation that takes into account changes in polymer relative viscosity that may take place before the spinneret. A meaningful operating condition is then adjusted in response to any deviations in relative viscosity. The present invention thus allows for relatively accurate control of yarn relative viscosity.